Hi guix,

since the update to icecat 68, mpeg decoding doesn't work in IceCat
(mp3/mp4 would not play, breaking a lot of online media players). This
patch addresses that issue, that was caused by IceCat not finding
ffmpeg's library. It was dlopening libavcodec.so, but could not find
it. I replaced it with an absolute reference to the library in the
store, which now allows IceCat to load the library at runtime. It also
adds ffmpeg to icecat's closure, ensuring it will always find it.